1. Field of the Invention
This invention relates to DC-to-DC converters which process electrical power from a source, at an input DC voltage, to deliver it to a load, at an output DC voltage, by selectively connecting a power transformer to the source and the load via solid state switches. In particular, the invention relates to converters of the forward type, in which the power transformer is simultaneously connected to the source and the load. More particularly, the invention relates to forward converters of the single ended type, in which the power flow from source to load is controlled by a single solid state switch.
2. Description of the Prior Art
This invention relates to the class of DC-to-DC converters which incorporate the topology represented in FIG. 1. A converter in that class is referred to as a "single ended forward" coverter because power flow is gated by a single switch 10 and energy is transferred forward, from the primary winding to the secondary winding of the transformer 11, during the ON period of the switch 10.
Converters in this class present a unique problem, in that the conversion topology does not inherently define the mechanism by which the transformer's core is to be reset during the OFF period of the switch. The solution to this problem is not unique, as evidenced by the multiplicity of proposals found in the literature which, in order to implement the reset function, complement the topology represented in FIG. 1 by differing choices of auxiliary circuitry. The differences are important since they affect the cost of the converter, as well as its efficiency and power density.
The traditional approach, represented in FIG. 2a, has been to reset the core via an auxiliary transformer winding connected with inverted polarity in series with rectifier 13. The operation of this reset mechanism is illustrated in FIG. 2b, where, in addition to idealized component behavior, a one-to-one turn ratio between auxiliary and primary windings has been assumed. This figure exemplifies a sequence of two ON periods, separated by an OFF period to enable the core to reset itself. The figure displays, as functions of time, the state of the switch 10, the voltage V across the switch, and the current I through the auxiliary winding.
The first ON period is given by the time interval between t.sub.1 and t.sub.2. During this interval, the voltage V across the switch 10 vanishes and the source voltage V.sub.o is impressed upon the primary winding. The magnetizing inductance controls the slope of the magnetizing current, flowing in the primary winding, and of the magnetizing energy which accumulates in the transformer's core. The current I vanishes, as the rectifier 13 is reverse biased, in a blocking state, and thus keeps the auxiliary winding inoperative.
At time t.sub.2, the opening of the switch 10 interrupts current flow in the primary winding. Neglecting the effects of leakage inductance between primary and auxiliary windings, the voltage V is clamped to 2 V.sub.o as the rectifier 13 becomes forward biased and begins to conduct the magnetizing current. The current I through the auxiliary winding is then equal to the peak value I.sub.p of the magnetizing current. Following time t.sub.2, I decays as magnetizing energy is returned to the voltage source V.sub.o. At time t.sub.3, the recycling of the magnetizing energy is completed, the current I vanishes, and, neglecting hysteresis, the magnetic flux through the transformer's core is reset to zero. The time interval between t.sub.2 and t.sub.3 is the core reset period. Having assumed a one-to-one primary to auxiliary turn ratio, this period equals the ON period t.sub.2 -t.sub.1.
The remainder of the cycle, between times t.sub.3 and t.sub.4, is the "dead" period. In this period, the switch 10 remains open, the voltage V across it equals the source voltage V.sub.o, and the current I vanishes. The circuit in FIG. 2a is not efficiently functional during dead time.
The relative duration of the dead period depends upon the duty cycle of the switch 10 (assumed to be 33% in FIG. 2b). At 50% duty cycle, the dead period vanishes. Operation beyond 50% duty cycle would lead to saturation of the transformer's core and (catastrophic) converter failure.
Thus, the traditional reset mechanism, represented in FIG. 2a, presents an inherent limitation in the available duty cycle range. This is a significant drawback as it impairs the ability of the converter to regulate against wide variations in the source voltage or in the load. Another drawback of the traditional reset method is that allowed values of the duty cycle are in general associated with a non-vanishing dead time. The existence of a dead time causes the switch 10 to experience a peak voltage greater than is in principle necessary to reset the core in the time interval between ON periods.
Similar limitations apply, to a varying degree, to any other reset mechanism which involves a variable dead time to accomodate variations in the switch duty cycle. Reset methods falling into this category are found in S. Hayes, Proceedings of Powercon 8, Power Concepts Inc. 1981 and in R. Severns, ibid..
To avoid these limitations, a different approach to the resetting of the transformer's core in single ended forward converters was proposed by S. Clemente, B. Pelly and R. Ruttonsha in "A Universal 100 KHz Power Supply Using a Single HEXFET", International Rectifier Corporation Applications Note AN-939, December 1980. These authors suggest a capacitor-resistor-diode network, as represented in FIG. 3a. The network clamps the switch to the minimal peak voltage consistent with a given source voltage and switch duty cycle, eliminating the need for dead time while allowing for a wide range of duty cycles. Attainment of these design goals is actually dependent upon component characteristics and values. In particular, the resistor 15 must be sized small enough so that the transformer's magnetizing current does not ever vanish.
With this assumption, the operation of this reset circuit is illustrated in FIG. 3b. As in the example given to illustrate the traditional reset mechanism, a sequence of two ON periods separated by an OFF period, with a 33% duty cycle is considered. The figure displays, as functions of time, idealized waveforms defining the state of the switch 10, the voltage V across it, and the current I through the rectifier 13. During the OFF period, the latter coincides with the transformer's magnetizing current.
As exhibited in FIG. 3b, the voltage V across the switch 10 is now a rectangular waveform with a peak value equal to 1.5 V.sub.o. The current I through the rectifier 13 is a trapezoidal waveform which, during the OFF period, decays from a peak value (I.sub.p +I.sub.o) to a minimum value I.sub.o, a non-negative function of V.sub.o and the duty cycle D.
A comparison of the voltage waveform of FIG. 3b with the corresponding one in FIG. 2b emphasizes the main advantage of the capacitor-resistor-diode clamp: a reduction in the voltage stress applied to the switch 10. A related advantage is the elimination of bounds resulting from core saturation on the duty cycle range, enabling the converter to remain functional over wider ranges of input voltage and output load. A further advantage is the avoidance of auxiliary transformer windings which simplifies transformer construction. Unfortunately, while attaining these benefits, the reset mechanism of FIG. 3a compromises the converter's efficiency and power density.
The reduction in the efficiency of the conversion process arises principally from the dissipation of magnetizing energy accumulated in the transformer during the ON period. Instead of being recycled, this energy is converted into heat by the clamp circuit. This power waste is significant, particularly in an otherwise efficiency mindful conversion system.
The reduction in power density results mainly from an increase in the size of the transformer which is rendered necessary by a decrease in the available dynamic flux swing for the magnetic material making up the transformer's core. This is evidenced in FIG. 3b by the quantity referred to as I.sub.o, which represents a non-negative, static component of the magnetization current. The component shifts the peak value of the magnetizing current, leading to an excitation of the transformer's core which brings the magnetic material closer to saturation. The consequent decrease in available flux swing reduces the power handling capability per unit volume at a given frequency of a given core and, with it, the converter's power density.
From these points of view, the traditional reset mechanism of FIG. 2a offers important, relative advantages. However, even from the point of view of core utilization, it does not represent an optimal reset mechanism, since the flux swing is still unipolar. This unipolar character of the transformer's core excitation has often been noted to be an inherent drawback of single ended forward converters. In fact, it is not a general drawback of this class of conversion topologies as it is only inherent to some reset mechanisms which have been adopted to complement those topologies.
These considerations suggest that the "optimal" reset mechanism for single ended forward converters, yet to be invented, should incorporate the following set of objectives:
it should be non-dissipative in nature, i.e. it should recycle the core's magnetization energy;
it should maximize the available flux swing, i.e. it should lead to a bipolar core excitation;
it should minimize the voltage stress on the switch, i.e. the voltage waveform should be rectangular without involving a dead period;
it should not introduce constraints on the switch duty cycle;
it should simplify transformer construction by eliminating the need for auxiliary windings.